1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic printer. More particularly, the present invention relates to a driving apparatus having a power disconnecting part which disconnects power transmitted to an image fixing unit while an image formed on an image transfer belt is being cleaned when a paper jam occurs, an image forming apparatus having such a driving apparatus, and a method for driving the same.
2. Description of the Related Art
In general, an image forming apparatus such as an electrophotographic printer forms an electrostatic latent image on a photoconductor such as a photoconductive belt or an organic photoconductive (OPC) drum, develops the latent image with developer having a predetermined color, and then transfers the developed image onto a recording paper, thereby obtaining a desired image. Such an electrophotographic image forming apparatus is classified into categories including wet type or dry type depending on the developer employed therein, wherein a wet-type uses liquid developer, formed by mixing powdered toner with a liquid carrier of volatile compounds, as the developer.
FIG. 1 shows a conventional wet-type electrophotographic color printer using a liquid developer.
As shown in FIG. 1, a wet-type electrophotographic color printer 1 comprises an image forming unit 5, an image transfer belt unit 10, an image fixing unit 21, a paper discharge unit 30, and a cleaning unit 50.
The image forming unit 5 comprises four laser scanning units 11, four charging rollers 12, four photoconductors 9, and four developing devices 13, to form an image of four colors, for example, black, yellow, cyan, and magenta.
FIG. 2 is a schematic view exemplifying a developing device and a photoconductor as shown in FIG. 1. As shown in FIG. 2, each developing device 13 comprises a storage part 6, a developing roller 7, a deposit roller 14, a metering roller 15, and a cleaning roller 16. The storage part 6 contains a reserve of liquid developer. The developing roller 7 is located below a corresponding photoconductor 9. The deposit roller 14 is located below the developing roller 7 and applies an electric force to the developing roller 7, thereby forming a layer of charged developer on the developing roller 7. Each metering roller 15 applies a predetermined level of voltage to the charged developer layer formed on the developing roller 7 by the deposit roller 14, so that a greater amount of toner is deposited on the developing roller 7. At the same time, the metering roller 15 limits the charged developer layer to a developer layer having a predetermined amount of toner or density (that is, solid %) and supplies the limited developer layer to a nip between the developing roller 7 and the conductor body 9.
The deposit roller 14 and the metering roller 15 serve to supply a layer of developer containing a constant amount of toner or toner density, regardless of a high density liquid developer 48 (for example, a denisity of about 3-40 solid %) or regardless of a fluctuating density of the liquid developer 48 (for example, a density fluctuated while being used), to the nip between the developer roller 7 and the photoconductor 9.
Each of the photoconductors 9 consist of an OPC drum, and the photoconductors 9 each form a different color toner image from one another, depending on the respective corresponding developing devices 13.
Returning to FIG. 1, the image transfer belt unit 10 comprises four first image transfer rollers 8, a second image transfer roller 23, and an image transfer belt 17. The image transfer belt 17 rotates along a path over an endless track on first, second, and third support rollers 19, 20, and 24. Each of the first image transfer rollers 8 transfers a toner image formed on a corresponding photoconductor 9 to the image transfer belt 17, and the second image transfer roller 23 transfers the toner image transferred to the image transfer belt 17 to a recording medium P.
The image fixing unit 21 comprises first and second heating rollers 25 and 27, and first and second compressing rollers 26 and 28. The first and second heating rollers 25 and 27 apply heat to the toner image transferred to the recording medium P, and the first and second compressing rollers 26 and 28 compresses the recording medium P against the first and second heating rollers 25 and 27 with a predetermined pressure. The recording medium P with the image fixed by the heat and pressure applied by the first and second heating rollers 25 and 27 and the first and second compressing rollers 26 and 28, is then discharged out of the printer 1 by the first and second paper-discharge rollers 31 and 32 and the first and second paper-discharge backup rollers 33 and 34.
The cleaning unit 50 comprises a cleaning blade 51 for removing an image remaining on the image transfer belt 17, and a waste developer storage part 52 for containing a reserve of waste developer removed by the cleaning blade 51.
The conventional wet-type electrophotographic printer 1 configured as described above is operated as described in greater detail below.
As a print command is applied, the image forming units 5 operate respective components thereof to perform a series of image forming operations for forming images of four colors.
Specifically, each photoconductor 9 is formed with a charged layer, that is, an electrostatic latent image corresponding to a color image to be printed, by a corresponding charging roller 12 and a corresponding scanning roller 11. A part of each photoconductor 9 having the electrostatic latent image is then deposited with toner from a developer layer having a predetermined amount of toner by a corresponding developing roller 7. The developing roller 7 communicates the toner from the liquid developer 48 of a corresponding storage part 6 using a corresponding deposit roller 14 and a corresponding metering roller 15, such that a toner image is formed on each photoconductor 9.
At this time, the liquid developer 48 is formed as a charged developer layer on the developing rollers 7 by the electric force exerted by the respective deposit rollers 14 and a predetermined level of voltage applied by the metering rollers 15, whereby the liquid developer 48 is formed on the developing rollers 7 as a developer layer containing a predetermined amount of toner.
The images developed on the respective photoconductors 9 by the respective corresponding developing devices 13 are primarily transferred to the image transfer belt 17 from the photoconductors 9 by the voltage and pressure exerted by the first image transfer rollers 8 located inside the path of the image transfer belt 17. The toner images transferred to the image transfer belt 17 are moved to the second image transfer roller 23 as the image transfer belt 17 is rotated along the first, second, and third support rollers 19, 20, and 24, by a belt driving roller 22. In a secondary transfer, the toner images are then transferred to the recording medium P by the voltage and pressure exerted by the second image transfer roller 23.
The images transferred to the recording medium P are fixed on the recording medium P by the first and second heating rollers 25 and 27, and the first and second compressing rollers 26 and 28, thus forming a desired image.
Thereafter, the recording medium P is discharged out of the printer 1 by the first and second paper-discharge rollers 31 and 32, and the first and second paper-discharge backup rollers 33 and 34, of the paper discharge unit 30.
After the images transferred to the image transfer belt 17 have been transferred to the recording medium P, the image transfer belt 17 is continuously rotated and arrives at the cleaning blade 51 mounted in such a manner that the cleaning blade 51 comes into contact with the image forming surface of the image transfer belt 17 at a side of the third support roller 24, wherein developer residue remaining on the surface of the image transfer belt 17 is removed from the image transfer belt 17 by the cleaning blade 51 and recovered by the waste developer storage part 52 so as to allow the printer 1 to print a next image. In a conventional printer, only 90-98% of the developer is transferred to a recording paper, rather than the entire 100%. Accordingly, the remaining 2-10% of the developer is collected by the cleaning blade 51.
The image transfer belt 17 then repeats the above-mentioned operations through the respective photoconductors 9, the respective laser scanning units 11, and the respective developing devices 13, after the remaining developer residue has been removed from the image transfer belt 17.
The conventional printer 1 described above is configured so that a paper jam can be easily removed through a user's simple actions, such as when a jam occurs in the printer as the recording medium P is caught or slipped during the secondary image transfer for transferring a toner image on the image transfer belt 17 to a recording medium P, or while the toner image is being fixed to the recording medium P. Therefore, when a jam occurs, a toner image remaining on the image transfer belt 17 that has not been transferred to the recording medium P, can be transferred to another recording medium P or transferred to the waste developer storage part 52 when the printer is operated again after the jam is removed.
However, when a serious jam occurs, such as an accordion jam or lap jam, which is difficult to remove by a user, the user will typically require a skilled engineer. In this event, a toner image remaining on the image transfer belt 17 that has not been transferred to the recording medium P is left as it is for a long time until the jam is removed by a repairman. As a result, the toner image remaining on the image transfer belt 17 that has not been transferred to the recording medium P is adhered to the surface of the image transfer belt 17 in a solidified form as the liquid carrier of volatile compounds contained in the toner image is evaporated to the atmosphere. Thus, even if the printer 1 is operated again after the jam is removed, the toner adhered in a solidified form to the image transfer belt 17 damages other components, such as the cleaning blade 51 and the photoconductors 9. As a result, a fatal problem may occur, which deteriorates the quality of a final image.
In order to solve this problem, there have been attempts to implement software in such a way that when a jam occurs for example, when a recording medium P is caught or slipped during a series of image forming operations, and if the jam is not removed within a predetermined period, the image transfer belt 17 is forcibly urged to rotate further, thereby removing the toner image remaining on the image transfer belt 17 that has not been transferred to the recording medium P.
However, such a printer 1 is usually configured so that the power of the driving motor (not shown) for driving the image transfer belt 17 is also transmitted to the first and second heating rollers 25 and 27 of the image fixing unit 21 through the belt driving roller 22.
Therefore, if the image transfer belt 17 is forcibly urged to additionally rotate so as to remove the jam, the jammed recording medium P may be torn or the jam may grow even worse, depending on the position of the jammed recording medium P. For example, in a case where an accordion jam or lap jam occurs when a toner image transferred to the image transfer belt 17 is being secondarily transferred to a recording medium P by the voltage and pressure of the second image transfer roller 23, or when an image transferred to the recording medium P is being fixed by the first and second heating rollers 25 and 27 and the first and second compressing rollers 26 and 28, if the driving motor driving the image transfer roller 17 is driven, the first and second heating rollers 25 and 27 and the belt driving roller 22 connected to the driving motor are jointly rotated. As a result, a jam occurring between the image transfer belt 17 and the second image transfer roller 23, or between the first and second heating rollers 25 and 27 and the first and second compressing rollers 26 and 28, may grow worse to such an extent that the user cannot solve the problem. If the jam becomes too severe, gear parts for transmitting the power of the driving motor may become immovably stuck together, thus causing damage to the driving motor, or the recording medium P may become burned by the image fixing heat from the first and second heating rollers 25 and 27, whereby a fire may occur. In each event, the problem again results in the toner becoming adhered in a solidified form to the image transfer belt 17 as the toner image is left as the printer 1 stands until the jam is removed as described above, which deteriorates the reliability of the printer 1.
In addition, with the conventional printer 1, when a recording medium P is jammed between the first and second heating rollers 25 and 27 and the first and second compressing rollers 26 and 28 of the image fixing unit 21 as the recording medium P is caught or slipped during the image forming operation, a force exerted on the recording medium P when a user pulls the recording medium P by hand so as to remove the jam, is transmitted to the driving motor through the first and second heating rollers 25 and 27 and the belt driving roller 22. Accordingly, the recording medium P can be torn or seriously damaged by the load of the driving motor without being easily removed.
Accordingly, a need exists for an image forming apparatus which can prevent a toner image that has not been transferred to a recording medium P from being left on an image transfer belt as a printer stands, such as when a jam occurs at the time of the secondary image transfer operation of the image transfer belt 17 or during an image fixing operation, and which can prevent the jam from growing worse even if the image transfer belt 17 is additionally rotated so as to remove the toner image remaining on the image transfer belt 17, and which still further allows easy removal of the jam.